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Spreading and Scrambling :

The pseudo random (PN) sequence is a bit stream of ‘1’s and ‘0’s occurring randomly, or almost randomly, with some unique properties. It is widely used in direct sequence spread spectrum wireless communication systems, for example, synchronous CDMA or asynchronous CDMA. The spread spectrum CDMA scheme is one in which the transmitted signal is spread over a wide frequency band, much wider than the minimum bandwidth required to transmit the information being sent. In general, the spread spectrum signals are commonly used for-

·        Combating or suppressing the detrimental effects of interference due to jamming, interference arising from other users of the channel, and self-interference due to multipath propagation;

·        Hiding a signal by transmitting it at low power density and, thus making it difficult for an unintended listener to detect it in the presence of background noise;

·        Achieving message privacy in the presence of other listeners.

·        Sharing the limited frequency resource to satisfy the demand of the increasing capacity in the public network

The desired properties of PN sequence depend clearly on the target application. Our goal is to study the code selecting in WCDMA system, the main emphasis is on the control of the interference arising from other users of the channel. The PN sequence selected should have high auto-correlation and low cross-correlation among different users’ PN codes.

            A block diagram of spreading and despreading signal in baseband of CDMA is shown in Fig .

            In transmitter, the spreader consists of multiplying the input data by a pseudo noise (PN) sequence, the bit rate of which is much higher than the data bit rate. This increases the data rate while adding redundancy to the system. The data bits modulated by spreading are called chips. The ratio of the chip bit rate to the data bit rate is called the spreading factor (SF). The resulting waveform is wideband, noise like signal .

Figure 5.1  Spreader Block Diagram

When the signal is received in the receiver side, the spreading is removed from the desired signal by multiplying with the same PN sequence that is exactly synchronised to the received chip level signal. When despreading is applied to the interference generated by other users’ signals, there is no despreading, this means that each spread spectrum signal should behave as if it were uncorrelated with every other spread signal using the same band. Therefore, CDMA codes are designed to have very low cross-correlation.